4aEA11. Measuring the acoustic reflectivity of panels at low frequencies
and high oblique angles.

Conventional short-pulse techniques for measuring the reflectivity of
acoustic panels requires that the reflected and incident waves be separated in
time so that their ratio can be established. This technique is limited to high
frequencies and low oblique angles, since under these extremes the direct and
reflected waves overlap. An interference method is investigated as a way of
making reflectivity measurements at low frequencies and high oblique angles.
The total field (incident and reflected) in the vicinity of the panel is
measured relative to the incident field without the panel. The total field,
which depends on frequency and angle of incidence as well as the panel
material, is then examined and manipulated to extract information on the
reflected wave. The technique is demonstrated with data reduced on steel plates
of various thicknesses.